Montana Earth Science Picture of the Week

This photo, taken about 20 miles southeast of
Great Falls (near Belt), shows an unusual
circular hole known as the Belt Meteor
Crater. Despite its name, the crater, which
measures 100 feet across and 35 feet deep,
was not made by a meteorite slamming into
the surface . . . In fact it has nothing to do with
rocks from space! Actually, the hole is a type
of formation that geologists refer to as a
sinkhole, and it was caused by changes that
took place in rocks that lie beneath the
surface.

Like Swiss cheese . . .
There are plenty of sinkholes in and around
the Little Belt Mountains of central Montana.
This is because a thick (up
to 1700 feet) formation called the Madison
limestone underlies the area. The Madison
limestone it is made up of microscopic
shells and other forms of calcite deposited
when shallow tropical seas covered much of
Montana during the Mississippian Period 350
million years ago. As water
soaks down through soils above the
limestone the water becomes slightly acidic.
Then as it works its way down through cracks
in the limestone, it dissolves away the calcite-
rich rock, forming caves. The Lewis and Clark
Caverns in the Madison limestone near Three
Forks were formed this way. As a result of this
process, the Madison limestone has so many
caves that it is often described
as a geological Swiss cheese.

Eventually these caves collapse, and that is
what happened to form the meteor crater
and other sinkholes. In the case of the meteor
crater, a fairly large cave must have formed in
the limestone not far beneath the prairie
surface. Eventually layers of
sandstone above the cave collapsed
onto the cavern floor to form the sinkhole. Watch the short slideshow below to see how this happens.

A deep blood kettle . . .
According to Dr. David Baker, the geologist
who guided me to the Belt Meteor Crater, it
once served as a buffalo jump, or "pishkun",
for Native
Americans as evidenced by
bison bones and arrowheads on the floor of
the hole. "Pishkun" is Blackfeet for "deep
blood kettle." Recently scientists visited the
sinkhole to collect bison bones that will be
carbon-dated to determine how long ago
Indians used it. They also found an
arrowhead(s) made of obsidian. Scientists
can determine where the obsidian came from
by comparing its mineral composition with
obsidian outcrops in the region. This will
provide insights about Native American trade
routes. Below: This is a Google Earth view of the Belt Meteor Crater, which is about 100 feet across and great rattlesnake habitat!

Below: The Monarch Sink . . .
An even larger sinkhole, called the Monarch
Sink, is located several miles south of Belt. It
is a circular hole that measures about 75 feet
deep and 150 feet across (too wide to fit in the
photo). Unlike the Belt Meteor Crater,
limestone at the Monarch Sink does not have
layers of sandstone above it. The huge rocks
scattered around me on the snow-covered
floor are pieces of Madison limestone that
made up the top of a very large cave before it
collapsed. The huge wall behind me is also
Madison limestone.

CLICK HERE to watch an ABC News video about a sinkhole in Florida (May 2012). Sinkholes are so common in Florida that some people purchase sinkhole insurance.

Photo by
Dr. David Baker

NOTE: Both the Belt Meteor Crater and the
Monarch Sink are located on private property.
Please do not trespass.